1. Field of the Invention
The present invention relates to novolak resin precursors for the preparation of novel novolak resins which are suitable as a resin ingredient of a positive photoresist composition, novolak resins, and methods of manufacturing these precursors and resins. The invention also relates to positive photoresist compositions containing the novolak resin.
2. Description of the Related Art
Positive photoresist compositions composed of an alkali-soluble novolak resin and a quinonediazide group-containing compound (photosensitizer) are used in practice as materials having high definition, sensitivity and etching resistance. In particular, those composed of an alkali-soluble novolak resin having moieties or units derived from m-cresol, p-cresol and 2,5-xylenol are known to suppress the formation of scum (residue after development) and to have satisfactory definition, sensitivity and other properties (Japanese Patent Laid-Open Nos. 6-202321 and 6-348007).
In such alkali-soluble novolak resins, the content of p-cresol unit affects definition and sensitivity. To be more specific, a large content of p-cresol unit improves definition and deteriorates sensitivity. Conversely, a small content enhances sensitivity and deteriorates definition. To obtain both satisfactory definition and sensitivity concomitantly, the use of a p-cresol-rich novolak resin having a low molecular weight (weight average molecular weight (Mw) of 500 to 5,000) has been investigated. This resin is, however, still insufficient in terms of the coating performance of a resultant resist composition relative to a substrate or the heat resistance of an obtained resist pattern.
On the other hand, it has been reported that the use of high ortho--ortho bonded novolak resins containing a large quantity of ortho--ortho bonding in positive photoresist compositions enhances definition (Japanese Patent laid-Open Nos. 59-162542, 60-159846, 4-202312, 6-345837 and 7-110576, SPIE, Vol. 631, pp. 76-82, 1986). These conventional high ortho--ortho bonded novolak resins are obtained synthetically from trifunctional phenolic compounds as raw materials, and their ortho--ortho (o--o) bonding is formed mainly between trifunctional phenolic moieties, as shown by the following formula. The term "trifunctional phenolic compound" used herein means a phenolic compound in which all of o- and p-positions relative to the hydroxy group of the compound are hydrogen atoms, and includes, for example, phenol and m-cresol. ##STR1##
As method of manufacturing the high ortho--ortho bonded novolak resins, these publications disclose (1) the synthetic process of reacting a trifunctional compound with an aldehyde using a divalent organic metal catalyst at a pH of 4 to 7; (2) the synthetic process of reacting a trifunctional phenolic compound with an aldehyde source in the presence of a nonmetallic inorganic or organic acid as a catalyst; and (3) the synthetic process comprising the steps of reacting a trifunctional phenolic compound with an aldehyde source at a temperature of from 100 to 200.degree. C. and at 2 to 15 atmospheres in the absence of a catalyst to thereby give an oligomer having a weight average molecular weight of 500 to 2,000; adding a mono-, di- or trifunctional phenolic compound and another aldehyde source to the oligomer, reacting the resultant mixture at a temperature of from 80 to 150.degree. C. to thereby obtain a high ortho--ortho bonded novolak resin containing 50 to 70% of ortho--ortho bonding and having a weight average molecular weight of 3,000 to 40,000.
The above-mentioned processes (1) and (2) cannot give high ortho--ortho bonded novolak reins having sufficiently high molecular weights, whereas the process (3) is unsuitable for mass production because of its special synthetic conditions. In addition, each of these novolak resins contains large quantities of binuclear compounds as by-products, which seem to be attributable to p-cresol having low reactivity, and the binuclear compounds volatilize in the step of drying a photoresist coating so as to stain the oven. Binuclear compounds can be removed by steam distillation, fractional precipitation or other techniques (Japanese Patent Laid-Open No. 7-110576), but such purification treatments are expensive, and complete removal of the compounds is still difficult.
In addition, all of the high ortho--ortho bonded novolak resins synthetically obtained by the aforementioned processes (1) to (3) are m-cresol/p-cresol novolak resins composed of trifunctional phenolic moieties (principally, m-cresol), and ortho--ortho bonding between trifunctional phenolic moieties, and resist compositions prepared by using these resins are disadvantageous due to their formation of scum.
It is, therefore, a principle object of the present invention to provide a novel novolak resin for a positive photoresist composition which contains less binuclear compounds, suppresses scum formation, has satisfactory definition and coating performance, and can give a resist pattern having excellent heat resistance; a precursor for the novolak resin; methods of manufacturing the resin and precursor; and a positive photoresist composition comprising the aforementioned novolak resin.